Unbalance correcting amplifier system



K. SINGER UNB ALANCE CORRECTING AMPLIFIER SYSTEM Dec. 11,1945.

' Filed Sept. 13, 1943 Kane?" Saves/e, INVENTOR.

BY I

EEZ

ATTORNEY.

. 11, 1945. K. SINGER UNBALANGE CORRECTING AMPLIFIER SYSTEM Filed Sept. 15, 1943 2 Sheets-Sheet 2 Annu- Ann. A.

I Ever Swa /e, INVENTOR.

ATTORNEY.

Patented Dec. 11, 1945 UNBALANCE CORRECTING AMPLIFIER SYSTEM Kurt Singer, North Hollywood, Calif., assignor to Radio Corporation oi America, a corporation of Delaware Application September 13, 1943, Serial No. 502,142

Claims.

This invention relates to electrical transmission systems and particularly to the automatic control of the gain or attenuation of such systems.

Electrical current transmission systems are well known, these systems generally having a linear relationship between the levels of the input and output currents. It has been found desirable, however, in certain types of transmission systems such as sound recording, radio, broadcasting, telephone communications, and public address systerns, to vary the gain or attenuation, both manually and automatically, in an amount depending upon the level of the original input signals.

When the output levels are varied downwardly from linearity with respect to the input levels, the system is known as a compressor, and when the output levels are varied upwardly from linearity with respect to the input levels, the system is known as an expander. Normally, compression is applied to a. selected upper range of input levels which may or may not require an expander before reproduction thereof. A commercial type of a compression system, wherein an expander is not required, is disclosed and claimed in my Patent No.. 2,255,683 of September 9, 1941.

The present invention involves the automatic control of the current levels loeing transmitted in the manner shown in my above-mentioned patent, and is directed to an improvement thereof to provide higher quality operation with less service maintenance. in the general use of such automatic level control systems, a pair of variable mu vacuum tubes connected in a push-pull relationship are employed, the signal voltages or currents to be compressed or expanded being applied to the control grids as transverse voltages, while a portion of the signal currents is rectified and applied to these grids as longitudinal voltages to vary the gain of the variable mu tubes. When the push-pull circuit is perfectly balanced, all applied longitudinal voltages will be balanced out and will not be introduced into the transversecircuit of the system and, therefore, no distortion from longitudinal voltages will result. However, it is impossible to maintain such a circuit in perfect dynamic balance at all times over the entire level range. One feature of the present invention, therefore, is to prevent longitudinal voltages from being introduced into the signal being transmitted when the push-pull circuit is dynamically unbalanced.

In compressor systems of the,type shown in the above-identified patent, a rectifier is usually employed to obtain a pulsating direct current procurrent. This pulsating current is filtered, to eliminate its alternating current components, and the voltage drop obtained when this current flows through a resistance is impressed longitudinally on the control grids of the variable mu tubes. The amount of filtering permissible in the circuit is limited by the required rapidity of the control portional to the amplitude variations of the signal action, and, in the past, a compromise had to be made between the two. The present invention provides for the cancellation of the alternating current components of the bias control currents without additional filtering. In'one modification of the invention, the control action is actually speeded up without deleterious effects.

The principal object of the invention, therefore, is to improve the automatic performance of a variable gain amplifier.

Another object of the invention is to provide an improved compressor circuit.

A further object of the invention is to provide a push-pull compressor circuit wherein any unbalance thereoi does not cause the introduction of longitudinal control voltages into the transmitted signals.

A still further object of the invention is to provide an automatic variable gain amplifier circuit wherein the longitudinal alternating current components are neutralized in the common portion of the grid-cathode and anode-cathode circults.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings forming a part hereof, in which:

Fig. 1 is a schematic diagram of a variable gain amplifier embodying one modification of the invention, and

Fig. 2 is a schematic diagram of the variable gain amplifier of Fig. 1 embodying a second modification of the invention.

Referring now to both figures wherein like elements are designated by the same numeral, a signal is impressed upon the primary of transformer 5 across the secondary of which are terminating resistances 6 and l. The secondary is connected to the control grids of variable mu tubes 9 and Ill connected in a push-pull relationship, the anodes of the tubes being connected .to a resistancecapacity network coupling the tubes 9 and iii to triode amplifier tubes 23 and 24. The coupling network comprises condensers l2 and I3 and resistors l4, l5, l6, and n. A potentiometer for balancing the circuit comprising resistors I9, 20,

and 2| and condenser 22 is connected across the v anodes of tubes 9 and Ill. I

The output circuit of amplifier tubes 23 and 24 is connected to the primary of transformer 25, the secondary feeding the output'circuit which may be either another amplifier, a recorder, a loud speaker, a modulator, or similar load. The secondary of transformer 25 is also connected by conductors 26 to the primary of atransformer 21. The secondary of transformer 21 is connected over potentiometer 28 to the input of an alternating current amplifier 30, the output of which is coupled through transformer 3! to a full wave rectifier 32. Anode potentials are supplied to the respective tubes from the plus and minus B terminals over conductors I and II and resistors 38 and 52, these resistors being shunted by bypass condensers 39 and 31, respectively. A bypass condenser 50 is also provided. Resistors 42 and 43 form a voltage divider for obtaining the proper screen grid voltage'for the tubes 9 and I0, condenser serving as a bypass condenser.

Referring now to Fig. l, the static operating bias for the tubes 9 and i0 is obtained by the voltage drop produced by the plate current flowsignal circuit. In the past, these types of compressor circuits have employed in the common portion of the grid-cathode and plate-cathode circuits, a resistor of a value suflicient to provide ing through inductance 45 and resistor'46, the

resistor 46 representing the direct current resistance of th inductance 45 and any additional resistance necessary to provide the proper bias. These elements are in the common portion of the plate-cathode circuitand the grid-cathode circuit, the latter circuit being from the control grids of tubes 9 and I0 over the secondary of transformer 5 and terminating resistors 6 and I, conductor 64, resistor 40, conductor 68, inductance 45, and resistor 46 to the cathodes of the tubes 9 and i0. Thus, any current change in the resistor 40 will vary the bias on the control grids of tubes 9 and I0 and vary the gain thereof. All currents flowing through resistor 40 are derived from the rectifier 32, the rectified current fiow from the rectifier 32 being from the cathodes of the rectifier through resistors and 36 shunted by condenser 33 of a potentiometer made up of resistors 34, 35, and 36, the connection to resistor 36 being variable to provide the proper level at which compression occurs as described in detail in my above-mentioned patent, through resistor and the secondary of transformer 3| to the anodes of the rectifier 32.

Shunted across, the resistor 40 is a condenser 41, the resistance of resistors 35 and 35, the resistance of the secondary of transformer 3!, and the internal resistance of the rectifier 32, determining the rate of charge of this condenser and the speed of operation of the gain change upon an increase in signal current. The resistor 40 determines th time of discharge of condenser 41 because of the high resistance of the rectifier 32, and consequently the rate at which the gain is restored to its previous value upon a decrease or cessation of the signal currents. The time of discharge of condenser 41 is considerably greater than the time of charge because of the high value of the discharge control resistor 40 with respect to the resistance in the charging circuit.

As pointed out above, since it is impossible to obtain a perfect dynamic balance in both halves of the push-pull circuit at all signal levels and the fact that the best balance obtainable does not ponents if they are not to be introduced into the the proper static bias for the variable mutubes. In other words, in place of the elements 45 and 46 shown in Fig. 1, a resistor has been used of a value of approximately 2200 ohms. This resistor provided a certain amount of feedback voltage, but an entirely insufiicient amount to prevent the introduction of the longitudinal currents into the signal when the push-pull circuit became out of balance to any appreciable degree. To permit obtaining sufilcient feedback voltage, the present invention utilizes an inductance such as shown at 45, and since this inductance may not include sufiicient direct current resistance to provide the proper static bias, a. building out resistor might have to be added. The value of inductance is determined by the amount of feedback action desired, which is a value to provide the proper neutralizing feedback voltage during gain changes taking place at a rate at which thump is introduced. This neutralizing action is also effective for the alternating current components from the rectifier 32. These thump and alternating current components would not, of course, be "introduced in a true dynamically balanced push-pull circuit without feedback. However, this feedback arrangement permits the circuit to function as though it were such a true dynamically balanced circuit. Thus, by the simple expedient of substituting an inductance for the normal static biasing resistor, the compressor circuit will tolerate a considerable dynamic unbalance without interference with its operation.

Although the modification shown in Fig. 1 and described above permits considerable unbalance of the variable gain circuit without deleterious effects, it does not permit of any increase in the speed of operation of the gain control action, and might even increase the acting time. To provide a system, therefore, with the same degreeof neutralization as provided by the inductance 45, and

which simultaneously permits faster operation than heretofore, the modification shown in Fig. 2 is used. In this modification only a resistor 6| is substituted for the normal resistor shown in my above-mentioned patent, the resistance of resistor 6| being in the neighborhood of 25,000 ohms, as compared with the normal resistance of 2200 ohms. It is obvious that the voltage drop across such a high resistor will provide too high a static bias voltage on the tubes 9 and II]. To permit the use of such a high resistance and simultaneously obtain the correct static bias, a certain amount of positive potential from the anode voltage source is applied to the grids to neutralize that portion of the direct current, voltage drop abovelthe required value.

In Fig. 2, therefore, a'variable resistor element 62 is added to the potentiometer connected across the anode voltage supply, the variable connection controlling the value of the positive voltage applied to the grids of tubes 9 and I 0 over conductor 65, resistor 60, and resistor 62. Thus, the modification in Fig. 2 has no tendency to slow up the gain control action of the compressor and in fact permits a more rapid gain change without additional filtering since the feedback voltage is obtained across a pure resistance. The modification does require, however, the use of an additional circuit to off-set the high positive bias'applied by the high resistance to the cathodes of the variable mu tubes.

in my above-mentioned patent in preventing the introduction of longitudinal currents into the si v nal when the push-pull circuit is considerably unbalanced dynamically, the modification of Fig. 2 actually permitting a speeding up ofthe control action of the system without additional filtering. Such compressors as described above, have operated over long periods withoutrequiring rebalancing.

I claim as my invention:

1. A system forobtaining a rapid control action of a variable gain amplifier comprising a push-pull circuit, a pair of vacuum tubes in said circuit, means for obtaining a direct current proportional to the variations in amplitude of a sig nal being transmitted by said tubes, a gridcathode. circuit for said tubes, a plate-cathode circuit for said tubes, said circuits having a common portion, means for obtaining a voltage proportional to the variations of said direct current for varying ,the gain of said tubes, said means bsing connected in said grid-cathode circuit, filter means for said direct current insufficient to completely eliminate alternating current components of said direct current and means in said common circuit portion for neutralizing said alternating current components introduced into said push-pull circuit due to the unbalance thereof. i

2. A variable gain amplifier system in accordance with claim 1 in which said last mentioned means comprises an inductance for producing a feedback neutralizing voltage and a static bias voltage for said tubes.

3. A variable gain amplifier system in accordance with claim 1 in which said last mentioned means comprises a resistor for producing a neutralizing voltage, additional means being provided for neutralizing the excessive positive bias on the cathodes of said tubes produced by said resistor.

4. A variable gain amplifier system having a rapid control action comprising an incoming line and an outgoing line, a push-pull circuit intermediate said lines including a pair of vacuum tubes, a rectifier for rectifyin a portion of the signal being transmitted by said tubes, a filter in the output of said rectifier for obtaining a voltage in accordancewith the variations in amplitude ofthe rectified current, said filter providing a rapid control action upon increases in amplitude of said signal but insufficient filtering to eliminate the alternating current components from said rectified current, an anode-cathode circuit for said tubes, a grid-cathode circuit for said tubes including said filter, said circuits having a circuit portion common to each, and means in said common circuit portion for neutralizing the alternating current components present in said rectified current and alternating current components produced in said plate-cathode circuit during sudden gain changes when said push-pull circuit is unbalanced.

5. A variable gain amplifier system in accordance with claim 4 in which said last mentioned means comprises an inductance of such a value rents in said plate-cathode circuit due to an unbalance of said push-pull circuit.

6. A variable gain amplifiercircuit in accordance with claim 4 in which said last mentioned means comprises 'a resistor having a sufficiently high value of resistance to produce a neutralizing' feedback voltage for the unbalance currents in the plate-anode circuit of said tubes, and in which means are provided for neutralizing the high positive bias on the cathodes of said tubes produced by plate current fiow through said high resistance.

7. A variablegain signal amplifier having a fast control action during increases in amplitude of a signal transmitted thereby comprisinga pair of vacuum tubes each having input electrodes and an associated input circuit and output electrodes and an associated output circuit, said input and output circuits being connected to form a pushpull amplifier system, means for applying to the input electrodes of said tubes a signal to be amplified, a rectifier for obtainin a direct current proportional to the amplitude variations of said signal, a filter for said direct current, said filter permitting said direct current to rise rapidly, but

contain undesirable alternating current components, means for applying to the input electrodes of said tubes a voltage derived from said direct current to vary the gain of said tubes in accordance with the amplitude of said signal, means for balancing said push-pull system, and means in said gain varying circuit of said tubes for maintaining said system in effective balance to eliminate the alternating current components in said direct current, and last mentioned means comprising an impedance the value thereof being such as to provide a neutralizing voltage to the input electrodes of said tubes in proportion to the currents in said output circuit produced by an actual unbalance of said system.

8. A signal amplifier in accordance with claim 7 in which said impedance is an inductance,

9. A signal amplifier in accordance with claim 7 in which said impedance is a relatively high resistance.

10. A circuit system for obtaining a fast aeting control for the gain of a variable gain amplifier having a pair of vacuum tubes in a pushpull circuit and a rectifier for rectifying a portion of the signal currents for obtaining a direct current, said circuit system comprising a filter in the output of said rectifier, said filter including a resistor through which rectified current fiows for varying the gain of said tubes in accordance with the variations in amplitude of said signal, said filter permitting the output current of said rectifier to rise rapidly but to contain alternating current compo-nents, an anode-cathode circuit for said'tubes, a grid-cathode circuit for said tubes, said anode-cathode circuit and said as to provide a neutralizing voltage said gridanode circuit, said voltage being caused by cur- 

